Observations of pCO2 in the coastal upwelling off Chile: Spatial and temporal extrapolation using satellite data

Abstract

Atmospheric and oceanic partial pressures of carbon dioxide and fluorescence were measured underway off the coast of northern Chile in January 1997. Seawater samples were taken for the analysis of nutrient and chlorophyll concentrations at 31 stations. The objectives were to improve the understanding of the biologically induced responses of upwelling processes off the coast of Chile and to assess the air‐sea exchange of CO2 during the austral summer. The cruise sampled within a grid from 22°40S to 24°S extending offshore to 71°52′W during 10–16 January 1997 (survey 1) and 22–27 January 1997 (survey 2). Survey 1 measurements showed relatively uniform hydrographic and chemical properties, except south of the Mejillones peninsula, where colder and fresher water was observed. During the second survey, concurrent satellite imagery showed colder water in a narrow coastal band. The relatively cold and fresh water recorded there suggested that upwelled water originated mainly from subantarctic water rather than from the nutrient‐rich equatorial subsurface water. Onshore‐offshore variability dominated over longshore variability. The offshore subtropical water was characterized by a temperature of 23°C, a salinity of 34.9, and relatively uniform pCO2 around 400 μatm that was mainly temperature driven. Although strong CO2 drawdowns (pCO2 as low as 200 μatm) were located close to the coast following the upwelling events, overall, the region was a source of CO2 to the atmosphere. A pCO2 algorithm was constructed using the cruise data of sea surface temperature (SST) and salinity (S). Synthetic salinity was calculated from S‐SST relationships determined for survey 2 and used with advanced very high resolution radiometer SST to calculate pCO2 over the cruise area, weekly, from January to March. European Centre for Medium‐Range Weather Forecasts wind speeds were used to calculate the CO2 flux in the upwelling off Chile (22°S–29°S, 68°W–73°W). The region was a source of CO2 flux of 1.6 mmol m−2 d−1 from 1 January to 25 March 1997.